Multi-layer printed circuit boards have found increasing use in the manufacture of electronic products. A typical multi-layer circuit board comprises a plurality of individual circuit boards laminated together. Each printed circuit board comprises an electrically non-conductive substrate material having conductive patterns formed on one or both sides thereof. The conductive patterns provide interconnect paths among the active and passive electronic components mounted on the board. In order to afford proper connections, predetermined points on the conductive paths on different individual boards must be interconnected, and these interconnections are typically made by drilling holes through the board at precise locations, followed by plating the through-holes with an electrically conductive material. Since a typical multi-layer board may have a large number of holes, the drilling typically is accomplished using a computer controlled automatic drilling apparatus. A typical drilling apparatus used for this purpose has multiple drill spindles which are independently activated by the master program in order to reduce the total drilling time required to form the multiple holes on a single multi-layer board. In order to properly position the multi-layer board initially in the drilling apparatus, some fixed referencing arrangement is usually employed, such as registration holes formed in edge portions of the board which mate with pins carried by the drilling apparatus table.
A problem encountered in the multi-layer board fabrication art is that of misregistration among the individual boards comprising the assembly. While the individual board patterns can be formed very precisely using conventional photolithography, exact registration among the multiple patterns on the several boards is impossible to achieve due to distortions introduced during the lamination processing. These distortions typically lead to maximum misregistration at the outer edges of the panels. The principal criterion for a useful board can be simply stated: each hole drilled through the multi-layer board must be surrounded by a conductive material at each layer in order to form a useful hole. Due, however, to the misregistration introduced during the lamination process, this criterion cannot be met by all multi-layer boards. In fact, the rejection rate for multi-layer boards has a present practical range of from 5% to 20%, depending upon the minimum pattern line width, maximum acceptable hole diameter, pattern complexity, and number of layers.
In the past, attempts at quality control for multi-layer circuit boards have centered about an inspection process wherein the developing multi-layer board is photographed at preselected stages of the fabrication process using an x-radiation sensitive film and an x-radiation source. After the films are developed, the successive photographs are compared to discern the degree of misregistration or distortion introduced during the intervening steps between the preselected stages. Once final multi-layer assembly is completed, and before the board is subjected to programmed drilling, a final comparison is made and the board is either accepted or rejected for drilling based upon this final comparison. Although useful, this process is slow and cumbersome and can only effectively be employed to sample representative multi-layer boards with theoretically identical patterns, which are undergoing multi-layer lamination. Since this technique is only amenable to spot sampling in a production environment, many multi-layer boards which should be rejected for misregistration or deformation may be passed on to the automatic drilling station, where they are uselessly drilled and ultimately scrapped. The automated drilling of a board which does not meet the minimum registration requirements is wasteful, since it results in a product which cannot be used. With relatively dense boards, thousands of holes may be drilled, which consumes relatively large periods of the drilling machine time. For example, in an 18 inch.times.24 inch multi-layer board, the number of holes typically ranges from 12,000 to 14,000, and the complete drilling of such a board can take as long as 90 minutes. As a result, the x-radiation source/x-radiation sensitive film inspection process has not been found to be a satisfactory solution to the problem of effective quality control for multi-layer boards prior to drilling.
In an effort to avoid the disadvantages with the x-radiation source/x-radiation sensitive film inspection process, a system has been developed to permit on-line inspection of multi-layer boards using an x-ray imaging system which examines test holes formed near the corners of the individual board layers and displays the percent of registration among all corresponding holes in a given corner region. While useful, this system is very large and expensive, and merely provides a percent registration figure for each set of test holes on a sequential basis. If a given multi-layer board falls within the permitted percentage of misregistration, it will be passed on for drilling. For those accepted boards which are close to the maximum permitted misregistration, the accumulation of tolerance errors inherent in the drilling machine can result in a multi-layer board with unacceptable through-holes.
The foregoing discussion of the prior art is taken largely from U.S. Pat. No. 4,790,694 to Wilent et al who propose registering a multi-layer printed circuit board prior to drilling by positioning the multi-layer board in an inspection fixture, examining a plurality of target areas located at predetermined locations on the multi-layer board with a radiation source and a detector, comparing the locations of the target areas with predetermined location coordinates, and marking the multi-layer board with reference indicia to provide proper positioning of the multi-layer board during drilling. Preferably, according to Wilent et al, the comparison is performed using predetermined location coordinates obtained by centering a master template having the target areas in the inspection fixture, and storing the location coordinates of the master template target areas. Further preferably, the marking step includes the formation of apertures along one edge portion of the multi-layer board.
While Wilent et al is believed to overcome certain of the deficiencies of the prior art as above discussed, Wilent et al employs relatively slow electromechanical servo-motors to move the master template in conjunction with another electromechanical device, the marking mechanism, which slows the process considerably. Moreover, the template itself is subject to warping, misalignment, and design tolerance errors. Finally, marking the multi-layer board with reference indicia in the form of apertures may result in connection lines being inadvertently commoned, shorted, or otherwise compromised.
Other problems common to multi-layer drilling systems include drilling inaccuracies due to drill bit wear and chipping, drill bit warding due to bit overheating, drill smear and machine down-time and rework costs due to drill bit breakage.
It is an object of the present invention to provide a novel method for fabricating a through-hole in a multi-layer printed circuit board which overcomes the aforesaid and other deficiencies of the prior art. More specific objects are to provide a novel multi-layer board registration and drilling method. Yet another object of the present invention is to provide a fabrication method that increases the quality of drilled through-holes and substantially decreases drill bit wear.